1. Field of the Invention
The present invention relates to optical polymerizable inkjet ink, an ink cartridge that accommodates the ink, and a printer in which the ink cartridge is installed.
2. Description of the Background Art
Typically, (meth)acrylic acid ester compounds, acrylamide compounds, oxetane compounds, and epoxy compounds are used as optical polymerizable monomers for use in optical polymerizable inkjet ink. Among these, (meth)acrylate compounds and acrylamides are preferable because these are widely available in significant quantities at low cost and can be used in combination with widely used, inexpensive radical polymerization-type optical polymerization initiators. However, most (meth)acrylic acid ester compounds and acrylamide compounds are toxic, and although some of them have relatively weak oral toxicity and dermal irritancy, the compounds having a low viscosity for use in inkjet ink can provoke an allergic response following contact with the skin.
In an attempt to find solutions to the problem of skin sensitization, several (meth)acrylic acid ester compounds and acrylamide compounds have been found that are suitable for use as optical monomers insofar as they have a Stimulation Index (SI) of less than 3 as measured by the Local Lymph Node Assay (LLNA) method. However, inkjet ink in which such compounds are blended to provide a curing property sufficient for practical use has a relatively high viscosity in comparison with typical optical polymerizable inkjet ink. Therefore, for example, if a desirable high image density is to be obtained by increasing the blend ratio of a pigment, such inkjet ink requires an extra decrease in the viscosity to achieve stable ejection.
In the optical polymerizable inkjet ink, a combinational use of a hydrogen-extracting type optical polymerization initiator such as a thioxanthone-based compound and a polymerization accelerator such as an amine compound is widely known, as described, for example, in Japanese Patent Applications Publications Nos. JP-H11-256085-A and JP-2006-28518-A. However, when these combinational uses are applied to polymerization of an optically polymerizable monomer having an SI of less than 3, the curing property of the inkjet ink is not satisfactorily improved.
It is possible to decrease the viscosity of the optical polymerizable inkjet ink by blending in an organic solvent, but such an organic solvent is volatile so that the viscosity of the ink increases, thereby degrading the stability of ink ejection. Moreover, such volatile organic compounds (VOCs) are emitted into the air, which causes an environment problem. Therefore, it is preferable to avoid using such VOCs.